The demand for wireless communication systems has grown steadily over recent decades, and has included several technological jumps over this time, particularly in the area of cellular and wireless local area network (WLAN) communication systems. Analogue cellular phones have been replaced with digital handsets using for example GSM and CDMA technologies, and so called third generation systems such as UMTS are now being introduced. Similarly WLAN technologies such as HyperLan and IEEE 802.11b are also being introduced. The number of users continues to increase and data traffic is now becoming an important part of the wireless network. Both of these factors mean that it is important for operators to look for methods of increasing the capacity of their networks to meet future demands.
As well as the need to increase capacity there is a general requirement to keep costs down whilst providing good performance. For example, costs for basestation and user terminal equipment should be reduced where possible whilst still enabling satisfactory wireless services to be provided.
One performance related problem relates to multipath fading. Typically basestations and user terminals are located in “cluttered” environments. This means that communications signals arrive at such basestations or user terminals via many paths because of scattering due to reflections and diffractions from buildings, furniture or other objects in the environment. Incoming scattered signals can add constructively or destructively depending on the relative amplitude and phase of the different components. This means that the received signal at the basestation or user terminal varies considerably in magnitude depending on the relative location of the basestation, user terminal and other objects in the environment. This effect is known as multipath fading.
Previously, one way of addressing multipath fading has been to use transmit or receive antenna diversity. Receive antenna diversity involves transmitting from one transmit antenna whilst providing two or more diverse receive antennas (e.g. with spatial or polarisation diversity). By using diverse antennas uncorrelated signals are received at those antennas. When one of those signals is in a fade the other is typically unfaded. In the case of switched antenna diversity, one of the receive antennas is selected for reception at any one time. Alternatively, adaptive combination is used in conjunction with all the receive antennas to produce one channel output. Thus in the ideal situation, the receive antennas can always be used to obtain an unfaded signal.
A similar situation occurs for transmit diversity. Here two or more diverse transmit antennas are used in conjunction with one receive antenna. Feedback about receive performance is used to either select one of the transmit antennas to use at a particular time, or to adjust adaptive combination of the transmit antennas to create one channel output. The present invention seeks to provide improved capacity and performance as compared with such known transmit and receive diversity antenna arrangements.
Another known approach for increasing capacity involves using multiple-input multiple-output (MIMO) communications systems to increase data rates. A MIMO wireless communications system (see FIG. 1) is one which comprises a plurality of antennas 10 at the transmitter 11 and two or more antennas 12 at the receiver 13. The antennas 10, 12 are employed in a multi-path rich environment such that due to the presence of various scattering objects (buildings, cars, hills, etc.) in the environment, each signal experiences multipath propagation. Thus a cloud shape 14 is shown in FIG. 1 to represent the scattered signals between the transmit and receive antennas. User data is transmitted from the transmit antennas using a space-time coding (STC) transmission method as is known in the art. The receive antennas 12 capture the transmitted signals and a signal processing technique is then applied as known in the art, to separate the transmitted signals and recover the user data.
MIMO wireless communication systems are advantageous in that they enable the capacity of the wireless link between the transmitter and receiver to be improved compared with previous systems in the respect that higher data rates can be obtained. The multipath rich environment enables multiple orthogonal channels to be generated between the transmitter and receiver. Data for a single user can then be transmitted over the air in parallel over those channels, simultaneously and using the same bandwidth. Consequently, higher spectral efficiencies are achieved than with non-MIMO systems.
However, one problem with known MIMO arrangements is that they are relatively expensive in the respect that multiple antennas are required together with multiple transmit and receive chains. One receive antenna is used for each MIMO channel. Thus, for example, a receive MIMO antenna arrangement can comprise four antennas together with four receive chains one for each of those antennas. Receive chains are relatively expensive, bulky and power must be provided to each of those receive chains. This is particularly disadvantageous for user terminals that are required to be compact and also for basestations which need to be unobtrusive. Similar problems occur for transmit chains.
An object of the present invention is to provide a radio communications device which overcomes or at least mitigates one or more of the problems noted above.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.